Though many of the toxic, mutagenic and carcinogenic effects of sunlight can be attributed to DNA photoproducts, the biological activities of individual photoproducts are unknown. To determine the precise structure-activity relationships, we have determined the structure of DNA photoproducts and developed building blocks for the site-specific incorporation of DNA photoproducts into oligonucleotides by automated synthesis. These uniquely photodamaged oligonucleotides are then used to construct: (a) short duplexes for high field 2D NMR and modeling studies, (b) polymers for DNA bending studies, (c) long DNA fragments for in vitro repair and replication studies, (d) viral DNA for in vitro mutation studies, and most recently (e) antigens for generating photoproduct-targeted catalytic antibodies, i.e., artifical repair enzymes. Mass spectrometry is critical for the structure elucidation of new photoproducts and characterization of synthetic intermediates and products used for these studies.